This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Amino Acids
• Organic compounds containing both the amine -NH2 and carboxyl -COOH functional groups.
• Amine e.g. CH3CH2NH2 ethylamine
• Carboxylic acid e.g. CH3COOH ethanoic acid.
• Simplest amino acid: aminoethanoic acid- glycine if you are a biologist.
• pH value at which the amino acid exists as a zwitterion.
• Varies from amino acid to amino acid since the inductive effects of the side chains affects the acid and base strengths of different amino acids differently.
• The isoelectric point has an impact on the acid-base behaviour of the amino acids.
Amphoteric Behaviour
• Amphoteric means can react with both acid and base .
• pH below isoelectric point:• The amino acid is a base and accepts a proton
from the the acid.• The amino acid is a positively charged ion.
• pH above isoelectric point:• The amino acid is an acid and donates a proton to
the base.• The amino acid forms a negatively charged ion.
Leucine
Isoelectric point pH 5.98
1. Draw the displayed formula of leucine.
2. Show the structure of leucine at pH 5.98, 2.0 and 7.
• Describe optical isomers as non-superimposable mirror images about an organic chiral centre.
• Identify chiral centres in a molecule of given structural formula.
• Explain that optical isomerism and E/Z isomerism are types of stereoisomerism.
24
• All molecules have a mirror image – but for most molecules it is the same molecule.
fluoromethane
H
CH F
H
H
CHF
H
25
• For some molecules the mirror image is a different molecule (the mirror image is non-superimposable). When this happens the 2 forms of the molecule are described as OPTICAL ISOMERS. Optical isomerism is a form of stereoisomerism.
OH
CH CH3
COOH
OH
CHH3C
HOOC
(-) lactic acid (+) lactic acidin sour milk in muscles
• Molecules that are optical isomers are called enantiomers.
• Enantiomers have identical chemical and physical properties, except:
• Their effect on plane polarised light;• Their reaction with other chiral molecules.
Many natural molecules are chiral and most natural reactions are affected by optical isomerism.
• In nature, only one optical isomer occurs (e.g. all natural amino acids are rotate polarised light to the left).
• Light is a form of electromagnetic radiation.
normal light(waves vibrate in all directions)
plane-polarised light(vibrates in only one direction)
plane-polarised light after clockwise rotation
• The wave vibrations are perpendicular to the direction of travel of the wave.
• Optical isomers rotate the plane of plane polarised light.
• Left and right hands are an example of non-superimposable mirror images.
Optical Isomerism
OPTICAL ISOMERISMOPTICAL ISOMERISM
Occurrence another form of stereoisomerism occurs when compounds have non-superimposable mirror images
Isomers the two different forms are known as optical isomers or enantiomers they occur when molecules have a chiral centre a chiral centre contains an asymmetric carbon atom
an asymmetric carbon has four different atoms (or groups)arranged tetrahedrally around it.
OPTICAL ISOMERISMOPTICAL ISOMERISM
Occurrence another form of stereoisomerism occurs when compounds have non-superimposable mirror images
Isomers the two different forms are known as optical isomers or enantiomers they occur when molecules have a chiral centre a chiral centre contains an asymmetric carbon atom
an asymmetric carbon has four different atoms (or groups)arranged tetrahedrally around it.
There are four different colours arranged tetrahedrally about the carbon atom
2-chlorobutane exhibits optical isomerism because the second carbon atom has four different atoms/groups attached
CHIRAL CENTRES
34
Most α amino acids are optically active – having non superimposable mirror image isomers. Exception?